The metal-producing industry continually faces the major challenge of increasing productivity, reducing costs, and maximizing benefits from existing equipment. Production of metals involves the basic steps of melting, processing and refining charges. During processing and refining, it is often critical that operating parameters are adjusted and controlled so the chemistry of the melt is within predetermined limits. Presently, charge compositions in many industrial processes are monitored by periodic sampling followed by time-consuming sample preparation and laboratory analysis. Virtually eliminating this delay through real time in-situ LIBS analysis has the potential to significantly increase productivity and improve process control. Other processes which, for example, involve the control and maintenance of alloys or non-metallic molten baths, such as used in the production of aluminum and magnesium, may also benefit from continuous monitoring of their elemental constituents.
LIBS can provide rapid, in-situ compositional analysis of a variety of materials in hostile environments, and at a distance. This technique involves focusing a high power pulsed laser on a material, thereby vaporizing and ionizing a small volume of the material to produce a plasma or spark having an elemental composition representative of the material. The optical emission of the plasma is analyzed with an optical spectrometer to obtain its atomic composition. Plasmas and sparks are used interchangeably in this specification.
A method for analyzing elements present in a sample using LIBS is known in the art. For example, a list of patents that are related to the technique can be found in U.S. Pat. 5,751,416, issued May 12, 1998 to Singh et al. Furthermore this method has been applied to a variety of materials and industrial environments. Unlike dealing with other liquids, LIBS analysis of high temperature molten materials in processing vessels often presents difficulties due to floating contamination or surface oxidation when the material is exposed to reactive atmospheres. To use the LIBS technique for analyzing molten materials and to overcome these problems, a method of data analysis is combined with means for exposing relatively unadulterated molten material surfaces for LIBS analysis. In the past, to address these problems and carry out LIBS measurements on molten material three approaches have previously been used, as exemplified in the following documents.
British Patent No. 2,154,315A, published Sep. 4, 1985 to Spenceley et al, describes a probe, which can be projected into the vessel to penetrate the surface of the molten metal below the slag layer. The probe is protected at its end by means of a ceramic collar suitably cooled and pressurized, to prevent damage by entry of metal, by a flow of inert gas entering the probe and exiting at a restricted port perpendicular to the probe, and parallel to the surface of molten metal. This approach can be applied only to a stationary stable surface. Further, in this configuration the laser irradiation samples an exposed surface which is not fresh and which is not necessarily representative of the molten metal. Furthermore, the laser irradiation is transmitted through a waveguide (optical fiber), which reduces the depth of field and restricts operation to short distances from the furnace.
U.S. Pat. No. 4,986,658, issued Jan. 22, 1991 to Kim, describes a probe for performing molten metal analysis by laser induced breakdown spectroscopy. The probe contains a high-power laser that produces a pulse with a triangular pulse waveform. When the probe head is immersed in molten metal, the pulsed laser beam vaporizes a portion of the molten metal to produce plasma having an elemental composition representative of the molten metal composition. The probe comprises a pair of spectrographs, each having a diffraction grating coupled to a gated intensified photodiode array. The spectroscopic atomic emission of the plasma is detected and analyzed for two separate time windows during the life of the plasma by using the two spectrometers in parallel. The spectra obtained during either the first or the second time window, or a combination of both, can be used to infer the atomic composition of the molten metal. In this configuration for obtaining an elemental composition representative of the liquid, the probe head must be immersed in the liquid or the molten metal. However, the immersed probe system is not easy to use and is not suitable for use with most molten metals or melt glass. Furthermore the probe samples a stationary surface which is not fresh and is problematic as explained above.
U.S. Pat. No. 4,995,723, issued Feb. 26, 1991 to Carlhoff et al, discloses a method and apparatus for optically coupling an element analysis system based on LIBS to the liquid metal in a melting vessel. A direct access to the slag-free metal bath is achieved through a bore hole in the sidewall of the vessel below the bath level or in the vessel bottom. To prevent liquid from escaping, a gas is blown in so as to produce the necessary counter-pressure. Again in this approach, the surface of the molten metal exposed to the laser irradiation is stationary. Furthermore it is difficult to prevent freezing of the surface.
Two temporally close sparks induced by two collinear lasers are used in U.S. Pat. No. 4,925,307, issued May 15, 1990 to Cremers et al, for the spectrochemical analysis of liquids. The laser light is not significantly absorbed by the sample so that the sparks occur in the volume inside the liquid. The spark produced by the first laser pulse produces a bubble in the liquid that remains in the gaseous state for hundreds of microseconds after the first spark has decayed. The second laser pulse, fired typically 18 microseconds after the first pulse, then produces a second spark within the gaseous bubble. The emission spectrum of the second spark, detected by a spectrometer oriented at 90 degrees to the laser beam axis, is thus much more intense and exhibits reduced line widths compared to the first spark, so that increased detectability of the atomic species is obtained by sampling the bubble with the second laser spark. This approach cannot be used for molten metals, opaque liquids or for real time measurement, as it is only suitable for off-line analysis of relatively transparent liquids.